When we view the defender as a "small sillohuette" large ship - it really seems to make sense in cost, and as to how it should fit in list building.
True - a small based ship is much harder to trap between rocks and other ships, like you can do with the spray, so its harder to hit. in gereral a small based ship is harder to shoot at compared to large-based ships.
Its hard to put this into account math-wise.